In most tensioning devices of this kind, the damping device is created by the presence of a leakage gap between the housing and the tensioning piston, where the leakage gap permits a controlled stream of leaking hydraulic fluid to flow to the outside. In these cases, the size of the leakage gap determines the expected damping characteristic of the tensioning device. These tensioning devices are typically connected to the engine oil hydraulic systems of a combustion engine, because these are primarily used on timing drives, in particular chain-based timing drives. This timing drive connects the crankshaft to one or several camshafts. In addition to the system based motor-oil throughput through the leakage gap, the disadvantage of this type of leakage gap damping also consists of the fact that the hydraulic fluid can incrementally escape through the leakage gap if the pressure reservoir is not pressurized. To limit the amount of escaping oil, the pressure reservoir is typically equipped with a packing. Moreover, most embodiments employ a helical compression spring between the housing and the tensioning piston. The compression spring provides a certain minimum pre-tension for the tensioning device, even when the pressure reservoir is not pressurized. Furthermore, air can accumulate in the pressure reservoir, in particular when the combustion engine is not in operation. This air would negatively impact the function of the tensioning device, because the air has a different compression behavior than the hydraulic fluid. For this reason, a venting device is proposed by which the air enclosed in the pressure reservoir can exhaust to the outside, in particular during the engine startup. Due to the packing, the oil quantity to be refilled is held to a minimum, for instance when the engine starts. But there are significant efforts underway to reduce the oil throughput through tensioning devices of this kind, not the least of which is driven by the desire to use smaller oil pumps.
The object of the present invention is therefore to provide a tensioning device of the initially described type that is able to reduce the hydraulic oil throughput during operation.
A tensioning device according to the class is configured in such a way that the damping device has at least one damping channel that is open in both flow directions as a component of the fluid supply, where at least a portion of the supply flow during operation is fed through the at least one damping channel into the pressure reservoir. Furthermore, the damping device and the venting device are coupled to each other in such a way that the drainage flow from the pressure reservoir occurs via the damping device and the venting device in combination, and the supply flow largely occurs independently from the venting device. At least a portion of the damping function is therefore provided by at least one damping channel (choke channel) that forces the hydraulic fluid back into the fluid supply during the damping process. The channel shape ensures that this process is choked and opposes the pressure of the oil supply. The oil volume used for the damping function is therefore not lost, but is readily available in the fluid supply to be fed into the pressure reservoir. The damping device and the venting device therefore do not typically form a structural unit, but instead operate in conjunction with the pressure reservoir in different locations. In the predominant number of cases, the venting action will occur on the tensioning piston in the known manner, while the damping device is directly connected to the hydraulic fluid supply via the damping channel. The venting device plays no significant role in feeding the supply flow. Of course, the venting device is equipped with a certain intake capacity for hydraulic fluid, which is sucked back into the pressure reservoir when vacuum is created in the pressure reservoir. But the venting device has no connection to the fluid supply in the true sense of the word. According to the invention, the inflowing share of hydraulic fluid due to the intake capacity of the venting device should not exceed 2%, preferably not exceed 1% of the total inflow volume into the pressure reservoir.
The venting device that is in a physical fluid connection with the pressure reservoir preferably vents independently from the at least one damping channel. In many cases the venting function is performed via the tensioning piston. For instance, designs are known with a vent bore arranged on the piston head of the tensioning piston, where said vent bore has a physical fluid connection to the pressure reservoir with the packing placed in between. To accomplish this, the packing can provide corresponding paths that are adjusted for a venting function. But the packing can also provide an additional damping function between the vent bore and the pressure reservoir, where paths and possibly storage cavities exist with sufficient volume to store hydraulic fluid. The designer then has several options on the tensioning device of influencing the damping properties, for instance on the supply insert and/or on the packing. A preferred method for this is the vent device on the tensioning piston. In accordance with the preferred embodiments, the share of the damping device of the total damping action of the tensioning piston should be at least 50%, preferably at least 60%.
Since at least a portion of the damping function occurs inside the tensioning device, and, for this purpose, no oil leaks to the outside that must be reconveyed by the oil pump, the gap optimization between the tensioning piston and the housing can be improved with respect to oil loss as a step toward furthering the design. This gap can play no role, or only a subordinate role, for any favorable advancements of the damping properties. For the purposes of the invention it shall be regarded as sufficient that the damping device according to the invention by means of the at least one damping channel as a component of the fluid supply shall have more than a 50% share of the combined total damping function of the damping device and leakage gap, with preference given to more than 80% and further preference given to more than 95%. Preference is given to the use of an economically sensible clearance fit between the tensioning piston and the housing, which must, however, provide an adequate seal regarding the leakage stream. An advantage in this regard is also that a tighter fit of this kind allows the use of a reduced guide length between the tensioning piston and the housing. Additional sealing can be provided, for instance by means of an O-ring. A significant advantage of the invention is that the required hydraulic volume is reduced because the fluid volume required for damping is at least partially available inside the tensioning device. This reduced hydraulic fluid consumption also addresses under-supply problems. Standardizing manufacturing dimensions for pistons and housings (in particular piston diameters and housing bore diameters) permits standardized piston housings of this kind for a variety of applications since the damping function can then be adjusted at least in part by the respective shape design of the at least one damping channel.
The venting device measures according to the invention must be differentiated. Said venting devices were specifically developed for the overhead installation of tensioning devices. For instance, EP 1602857 B1 reveals a tensioning device with a specially configured check valve housing that contains a choked bypass for venting into the oil supply channel. While such a configuration may also have an impact on the damping properties of the tensioning device, the supply of new hydraulic oil into the pressure reservoir is most likely exclusively provided via the open check valve due to the adjusted choke effect of the medium air. A similar venting design for overhead tensioning device installations is also disclosed by U.S. Pat. No. 5,643,117. According to this document, a special choke disc is used as a check valve mount. However, in both known designs, new hydraulic fluid is supplied primarily through the open check valve. No involvement of the venting design in this inflow is described. In contrast to these known designs, the invention also permits embodiments where the tensioning piston presses toward the top. In addition, in this known design, the exhausted air remains in the pressurized supply path, which can lead to problems.
In accordance with a preferred embodiment, the fluid supply can be equipped with a supply insert that is inserted into the housing with an accurate fit, where the at least one damping channel is arranged between the supply insert and the housing. By employing such an insert, the at least one damping channel can be easily created and also varied. This permits the use of simple methods to influence the damping properties of the damping device by employing a supply insert that matches the desired application.
The housing can be preferably configured with a receiving bore in which the tensioning piston can be located in a sliding manner, where the supply insert is seated with an accurate fit at the base of the receiving bore, and the at least one damping channel is arranged between the exterior surface of the supply insert and the interior surface of the receiving bore. This permits the supply insert to be conveniently inserted into the receiving bore from the front before the tensioning piston is inserted into the housing. The insertion with an accurate fit permits the easy attainment of a suitably sealed damping channel. Ideally, the supply insert could be pressed into the receiving bore.
Yet another embodiment envisions that the supply insert is configured with a first section and a second, larger diameter section, where the at least one damping channel is shaped into the second section, and a flow gap is formed between the first section and the housing, where the flow gap is a component of the fluid supply. This permits the receiving bore diameter to remain unchanged, while the shape of the supply insert creates a flow gap that feeds the hydraulic fluid to the at least one damping channel. Shaping the at least one damping channel into the supply insert simplifies the manufacturing process as a whole. For instance, this certainly provides for the option to manufacture the supply insert as an injection molded part.
Furthermore, the housing can feature a supply bore that is connected to an external oil supply as a component of the fluid supply and terminates in the flow gap between the housing and the supply insert. This can be in the form of a simple cross bore to the receiving bore for the tensioning piston, so that the supply insert is primarily responsible for defining the essential shape of the adjoining flow channels.
In order to achieve sufficient damping and to provide damping channels of sufficient length, the at least one damping channel can run circumferentially along the outside diameter of the supply insert in a helical manner. Depending on the selected pitch, a partial circumferential design may be completely sufficient in this case.
A minimum of two damping channels on the supply insert is the preferred embodiment, where the regions between the damping channels provide a nearly complete contact seal with the housing. This ensures that the damping action is accurately determined by the damping channels, and that no leakage flow occurs past the supply insert.
In accordance with a preferred embodiment, all supply streams originating from the oil supply can be supplied by the damping channel into the pressure reservoir. The term “all supply streams” refers to the hydraulic fluid supply stream fed from outside of the tensioning device. Other internal storage mechanisms may additionally be employed inside the tensioning device. Conversely, this means that the supply of the pressure reservoir with hydraulic fluid is performed without a check valve because the damping channel must be available at all times for the damping and the supply functions. The tensioning device must therefore be regarded as devoid of a check valve for this supply purpose within the fluid supply area. The damping properties are naturally adjusted by selecting the appropriate number, size, cross-section shapes and trajectory shapes of one or several damping channels.
Alternatively, an embodiment can also be equipped with a fluid supply that has a check valve that is actuated hydraulically in parallel to the damping channel. Depending on the configuration of the check valve, the supply of hydraulic fluid into the pressure reservoir can occur very quickly and with relatively low flow resistance, while very good damping action is provided by means of a damping channel that only provisions against a large flow resistance during the retraction stroke of the tensioning piston, during which the check valve is closed.
It is therefore advantageous for such a version if the inflow into the pressure reservoir primarily occurs by means of the check valve. To do so, a design can be envisioned where the flow resistance of the at least one damping channel and the flow resistance of the check valve are jointly adjusted in the opening direction in such a way that at least for a tensioning piston stroke frequency range of 50 to 200 Hz more than 90%, preferably more than 94%, of the supply flow is provided to the pressure reservoir via the check valve. The tensioning piston stroke frequency was referenced for the above consideration because this also incorporates the dynamic behavior of the check valve into the consideration. Namely, as the stroke frequency increases, the inflow via the check valve is reduced slightly, whereas the inflow via the at least one damping channel is increased slightly. For example, in one version at a stroke frequency of 50 Hz, the share of the supply flow through the check valve is at 97%, whereas it is only 95% at 200 Hz. Correspondingly, the share of the supply flow through the at least one damping channel changes from 3 to 5%.
Furthermore, a design can be envisioned where the flow resistance of the at least one damping channel and the flow resistance of the venting device are jointly adjusted in such a way that at least for a tensioning piston stroke frequency range of 50 to 200 Hz, no more than 50%, preferably no more than 45%, of the drainage flow can be evacuated from the pressure reservoir via the venting device. The damping action should therefore primarily be determined by the at least one damping channel (damping share greater than 50%). But due to the dynamic behavior of the check valve, said check valve participates in the drainage flow as the tensioning piston stroke frequency increases. Generally, at this tensioning piston frequency range, the share of the check valve is less than the share of the at least one damping channel, as well as the share through the venting device, but the check valve nevertheless plays a noteworthy role for the overall consideration across the range indicated herein. The share of the ventilating device declines with increasing tensioning piston stroke frequency in the indicated range (starting at 50 Hz), therefore initially resulting in an increase of the share of the drainage flow through the at least one damping channel. In many cases, this is likely to apply up to a stroke frequency of 100 Hz. Beyond this, the check valve increasingly assumes a share of the drainage flow, so that the relative share of the drainage flow that is evacuated through the damping channel declines again. There certainly are versions for which the volume flow evacuating via the check valve is 13% of the entire drainage flow at a tensioning piston stroke frequency of 200 Hz. But at low frequencies, the adjustments between the at least one damping channel and the venting device is of material significance, since these distribute the damping action share among each other. However, the share of the at least one damping channel is mostly greater than 50% at the stroke frequency range indicated here. This ensures that even if a check valve is used, the correspondingly desired savings effects regarding the oil throughput are attained.
An advantageous embodiment envisions that the supply insert is manufactured from a material softer than the material of the tensioning piston. By selecting a suitable material, the supply insert can also serve as an impact damper for the tensioning piston. This would limit or possibly avoid undesirable noise. Moreover, the material of the supply insert can also be softer than that of the housing. This permits the supply insert to be pressed into the receiving bore, without the receiving bore being subject to undesirable surface modifications or damage to the interior surface.
Based on a particularly favorable embodiment, the damping function by means of a leakage gap between the tensioning piston and the housing is omitted completely. For this purpose, a slip seal can be arranged in the housing between the tensioning piston and the receiving bore. The damping function can then be performed completely via the at least one damping channel, or additional measures can be taken, e.g. due to a specially configured packing, which also contributes to the damping function.
The invention furthermore refers to a tensioning device series, encompassing at least one first and at least one second tensioning device in accordance with one of the claims 2 through 12, where the first and second tensioning devices each have a housing bore to accommodate the tensioning piston and have a tensioning piston of an identically dimensioned standardized diameter, and the damping properties of the damping device of the at least one first tensioning device differ from those of the at least one second tensioning device, where the differing damping properties are achieved by means of differently dimensioned and/or designed supply inserts.
A most favorable outcome for such a tensioning device series is that the housing and the tensioning piston are correspondingly identical in their configuration, while primarily a different supply insert is used for the respectively desired damping properties. This can be taken so far that all components of the first and second tensioning devices are identical with the exception of the supply insert. This could result in significant cost reductions, while the damping function can be individually adapted to various application requirements. The damping function can be adjusted by exchanging the supply insert, particularly when employing timing chain drives on combustion engines. Such a measure represents a significant advantage, given the cost pressures typical for this field. Moreover, the supply insert of this tensioning device exhibits the low to non-existent hydraulic fluid losses expected from the design according to the invention. Of course, the series can be extended at will, so that the series can contain even more than two tensioning devices with different damping functions.
In addition, the invention refers to a circumferential engagement device transmission with a flexible drive device, such as a chain or belt, at least two drive wheels that are functionally engaged with the drive device, and a tensioning device in accordance with one of the claims 1 through 12. In the case of a chain drive, the tensioning device applies pressure to a pivoting tensioning bar that contacts the chain, thereby tensioning the chain between the transmission gears. For a circumferential engagement device transmission of this type, the supply of hydraulic fluid is simpler than in the comparable state of the art.
The invention also refers to a circumferential engagement device transmission series, encompassing at least one first circumferential engagement device transmission and at least one second circumferential engagement device transmission, where the first circumferential engagement device transmission is equipped with a first tensioning device of a tensioning device series in accordance with claim 13, and the second circumferential engagement device transmission is equipped with a second tensioning device that has a different damping characteristic of a tensioning device series in accordance with claim 13. Costs can be reduced in this case by employing standardized components. But the advantages of the employed tensioning devices also have a positive impact on the operation of the circumferential engagement device transmission series.
The invention can be combined with other measures to reduce oil throughput. For instance, the venting device can provide expanded storage capacity for hydraulic fluid. An advantageous embodiment for this could be a corresponding design of a packing in the interior of the tensioning piston, so that a suitable venting action and hydraulic fluid storage capacity can be rendered at the same time. The venting device then typically also acts increasingly as a damping device. In the most favorable case, the hydraulic fluid travels back and forth in the working space of the tensioning device within the venting device, without an inordinate amount of hydraulic fluid leaking via the venting device opening. This requires exacting tuning of the choke channels and/or storage reservoirs within the venting device, especially in consideration of the damping device in accordance with this invention.
In addition to this, the option exists to create different configurations. For instance, the venting device can be equipped with a pressure relief valve that opens at a certain pressure level in the pressure reservoir in order to relieve pressure peaks. Typically, the venting action is rendered as a bypass to a spring-loaded valve body that provides the pressure relief function. Alternatively, or additionally, a reverse lock can be provided on the tensioning device, which prevents the retracting motion of the tensioning piston beyond a certain point. Typically, these reverse locks are self-adjusting devices in the form a ratchet system, in order to compensate wear in the flexible drive devices and to effect a shift of the operating parameters of the tensioning device.